11 Effect of Plasticization on Viscoelastic Properties of Polyvinyl Chloride M . C. S H E N a n d Α. V. T O B O L S K Y
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Princeton University, Princeton, N. J.
M o d u l u s - t e m p e r a t u r e a n d m o d u l u s - t i m e curves up t o h i g h t e m p e r a t u r e s w e r e m a d e for poly v i n y l chloride (PVC) a n d polyvinyl chloride p l a s t i c i z e d w i t h 30% d i o c t y l p h t h a l a t e . Incor p o r a t i o n o f s t a b i l i z e r and rapid m e a s u r e m e n t s m i n i m i z e d c h e m i c a l d e c o m p o s i t i o n and d i l u e n t e v a p o r a t i o n . At t e m p e r a t u r e s a b o v e the melt i n g p o i n t o f P V C c r y s t a l l i t e s , flow r e g i o n s are o b s e r v e d . The p r e s e n c e o f p l a s t i c i z e r c a u s e d a l o w e r i n g o f T a n d of T . B e l o w T b o t h p u r e PVC a n d plasticized P V C b e h a v e like semicrystalline polymers. The presence of diluents affects t h e viscoelastic p r o p e r t i e s b e l o w T m a i n l y b y c h a n g i n g T o f the a m o r p h o u s re g i o n s , w i t h o u t v e r y significant c h a n g e s in the m i c r o c r y s t a l l i n e structure. A b o v e T t h e re laxation t i m e s are d i f i n i t e l y s h o r t e n e d b e c a u s e o f the l o w e r v a l u e o f T . The effect m a y b e e v e n g r e a t e r than t h i s b e c a u s e the over-all a v e r a g e m o l e c u l a r w e i g h t o f p o l y m e r plus p l a s t i c i z e r is l o w e r t h a n t h a t o f p o l y m e r a l o n e . m
g
m
m
g
m
g
polyvinyl chloride (PVC) is one of the more important of the polymers that are frequently used in the plasticized form. Its superior mechanical behavior has been the active interest of many investigations. Previous reports from this laboratory have dealt with x-ray, dilatometric, and birefringence properties of plasticized ajid unplasticized polyvinyl chloride (2, 25). Ferry et al. studied the dynamical mechan ical properties of PVC gels (5, 10, 11, 12, 20). Creep (/, 9, 17) and elastic (33) be havior have also been investigated. In recent publications (27, 30), we have re ported time-dependent viscoelastic properties of PVC covering a range of plasticizer contents. An important conclusion emerging from these studies is that polyvinyl chloride has a three-dimensional network structure where microcrystallites are believed to 118 In Plasticization and Plasticizer Processes; Platzer, N.; Advances in Chemistry; American Chemical Society: Washington, DC, 1965.
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11.
Viscoelastic
SHEN A N D TOBOLSKY
119
Properties
act as cross linkages (2, 25). Crystallinity persists even at relatively high extents of plasticization. Consequently, moreflexiblepolymers are obtained by plasticization without losing the resistance to flow. Recently, further x-ray studies by Natta and Corradini (22) revealed that radical-polymerized polyvinyl chloride possesses imperfect syndiotactic structure. Laterally ordered crystallites of the order of 50 A. were present. Polyvinyl chloride has been known to decompose at high temperatures (> 150°C.) with the evolution of hydrogen chloride (4, 6,15). It has been suggested that abstraction of hydrogen chloride from neighboring chains may have produced chemical cross-links giving the observed high creep recovery property. However, evidence from chemical decomposition indicates that removal of hydrogen chloride occurs within the same chain (18). To reaffirm the hypothesis that polyvinyl chloride is semicrystalline, viscoelastic properties at high temperature were studied. Regions of rubberyflowshould be observed for PVC above the melting point of its microcrystallites. Expérimentai Methods Samples were kindly supplied by Allied Chemical Corp. The unplasticized polyvinyl chloride contained 4.7 parts by weight of stabilizer and lubricant per 100 parts of PVC. The plasticized sample had 40 parts of dioctyl phthalate and 2.5 parts of stabilizer per 100 parts of polymer which is equivalent to a total plasticizer content of ~ 30 weight %. Two types of measurements were made on these samples. In the region where moduli are higher than 10 dynes/sq. cm., a Clash-Berg torsional creep apparatus (7) was used. For moduli below 10 dynes/sq. cm., a modified Gehman apparatus (14) was employed. In both cases shear creep compliance, J (t), was obtained. To convert this to relaxation modulus, £>(/), the following equation was used : 9
9
e
G (/)= -^-itn τ J (t) r
(1)
S
e
where m is the slope of the double logarithmic plot of J (t) versus t. Data were presented as 3G (t) which is approximately equal to Young's modulus. Parallel measurements were made on a stress-relaxation balance (29). Good agreement was obtained. A Tenney environmental test chamber (Model TSU-100) was used as an air bath. Temperature can be controlled to =fc0.1°C. by adapting an Aminco bimetallic thermoregulator connected to a supersensitive relay. Samples were prepared in the form of rectangular strips. First, moduli were measured as a function of tem perature. At leastfiveminutes was given to each temperature increment (~5°C.) to obtain thermal equilibrium. A series of modulus-time curves was also made for temperatures covering the entire viscoelastic spectrum. At higher temperatures, the instrument was first allowed to reach the desired temperature and was held there for half an hour. Samples were then quickly inserted. After 10 minutes moduli were measured as a function of time up to 1000 seconds. Total measuring time for each isotherm thus was kept to less than 30 minutes to minimize chemical decomposition and plasticizer evaporation. c
r
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In Plasticization and Plasticizer Processes; Platzer, N.; Advances in Chemistry; American Chemical Society: Washington, DC, 1965.
120
PLASTICIZATION A N D PLASTICIZER PROCESSES
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Results and Discussion In all previous work, viscoelastic measurements were performed at relatively low temperatures, below the occurrence of rubbery flow. In the work reported here viscoelastic properties of PVC were studied at temperatures higher than the melting point of the crystals. As mentioned in the previous section, precautions were taken to minimize decomposition by incorporating stabilizers and by rapid measurements. However, slight yellowing of unplasticized samples were observed after measurements, indicating some decomposition. For plasticized samples, there were minor losses of weight (~1%) by the end of experiments at high temperature, perhaps caused by evaporating plasticizer. However, the essential features of viscoelastic behavior is not believed to have been profoundly affected. Thus we feel that conclusions drawn from our experimental results can be accepted with reasonable confidence. If indeed the degree of cross linking can be increased by heating, as suggested by Doty and Zable (
